Many places sell ATmega chips with the Arduino bootloader pre-loaded. To actually burn the bootloader yourself, you do need an external programmer.  Fear not though – in a previous post, I discovered how to use an Arduino board (together with the mega-isp project) as a make-shift external programmer -> see this post for details.

Putting the Boot-in..

The version of the bootloader that you need to use, depends on the type of hardware that you intend to program.  The Arduino IDE supplies versions of the bootloader for a range of ATmega / Arduino combinations .. see the ‘./hardware/bootloaders/’ sub-directory within your arduino installation directory for more info.

There are also custom versions of the bootloader created by third-parties -> ladyada has produced some bootloader hacks and another unified version of the bootloader can be found here.

Atmel's ATmega8 microprocessor

Once you’ve decided which bootloader you want to use, you can use AVRdude to upload the code.

Steps Required to Burn the Bootloader

1. Unlock the bootloader segment
2. Upload the bootloader code
3. Relock the bootloader segment

Step 1: Unlock

First of all, it’s a good idea to make sure that you can communicate with your programmer successfully. To do so, you can issue a simple command via AVRdude, which will give you a nice ‘all clear’ status message as long as everything is set-up correctly.

The parameters that you use might vary according to your system settings and the external programmer you’re using.

avrdude -p m8 -P /dev/ttyUSB0 -c avrisp -b 19200

Briefly, ‘-p’ defines the chip I’m programming, ‘-P’ is the port that my programmer is connected to, ‘-c’ is the programmer type and the baud-rate is specified by ‘-b’.  You can run ‘avrdude –help’ to discover the full range of options.

This returned:

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.13s

avrdude: Device signature = 0x1e9307

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

.. which is good.  This means that the stage is set for programming and we can begin to set the fuses to unlock the bootloader segment of the AVRdude.

Each ATmega chip has a number of Fuses which can be set.  I think of the fuses as configuration bits -> they define the way that the chip is going to function on a very low level.  I’ve been using a fuse-calculator to simplify the process, and I’d suggest that you do the same.

avrdude -p m8 -P /dev/ttyUSB0 -c avrisp -b 19200 -e -U lock:w:0x3F:m -U lfuse:w:0xc4:m -U hfuse:w:0xd9:m

The ‘-U lock:w:0×3F:m’ performs the unlock and ‘-U lfuse:w:0xc4:m -U hfuse:w:0xd9:m’ is used to set the chip’s low and high fuse values.

I want to use this particular ATmega chip as a minimal Arduino, so I’ve set the fuses to instruct the chip to use it’s own internal 8 MHz oscillator rather than an external crystal oscillator.

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.13s

avrdude: Device signature = 0x1e9307
avrdude: erasing chip
avrdude: reading input file "0xc4"
avrdude: writing lfuse (1 bytes):

Writing | ################################################## | 100% 0.04s

avrdude: 1 bytes of lfuse written
avrdude: verifying lfuse memory against 0xc4:
avrdude: load data lfuse data from input file 0xc4:
avrdude: input file 0xc4 contains 1 bytes
avrdude: reading on-chip lfuse data:

Reading | ################################################## | 100% 0.04s

avrdude: verifying ...
avrdude: 1 bytes of lfuse verified
avrdude: reading input file "0xd9"
avrdude: writing hfuse (1 bytes):

Writing | ################################################## | 100% 0.04s

avrdude: 1 bytes of hfuse written
avrdude: verifying hfuse memory against 0xd9:
avrdude: load data hfuse data from input file 0xd9:
avrdude: input file 0xd9 contains 1 bytes
avrdude: reading on-chip hfuse data:

Reading | ################################################## | 100% 0.04s

avrdude: verifying ...
avrdude: 1 bytes of hfuse verified

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

Step 2: The Burn

The chip has now been set up – the next stage involves uploading the new firmware.

avrdude -p m8 -P /dev/ttyUSB0 -c avrisp -b 19200 -D -U flash:w:/opt/arduino/hardware/bootloaders/atmega8/ATmegaBOOT.hex

… which produces …

Step 3: Re-lock

The final stage involves re-setting the lock fuse:

avrdude -p m8 -P /dev/ttyUSB0 -c avrisp -b 19200 -U lock:w:0x0F:m

And that’s it!

Programming the ATmega

It’s possible to program Atmel microprocessors using an Arduino board and the megaISP project.  This post documents my attempt.

Before starting of got hold of a copy of the datasheet for the ATmega8 (http://www.atmel.com/dyn/resources/prod_documents/doc2486.pdf)

Step 1: Upload mega-isp to Arduino

I downloaded the mega-isp source, and uploaded it to my Arduino board.  The source can be found at google code (http://code.google.com/p/mega-isp/).

Step 2: Set up breadboard

mega-isp: programming an atmega8 with arduino

First of all, I set up the breadboard according the advice given at http://www.uchobby.com/index.php/2007/11/04/arduino-avr-in-system-programmer-isp/

The article mentions something called a bypass capacitor .. which is something I hadn’t come across before.  Basically, it’s a small capacitor that’s used to smooth out small changes in voltage – and should be used whenever you set up a microprocessor in a circuit.  I learnt about them here (http://www.seattlerobotics.org/Encoder/jun97/basics.html)

Because I’m trying to program an ATmega8 (and not the Tiny18 mentioned in the  article) I needed to adjust the pins.  I also added another LED to signal when megaisp is actually programming (which was mentioned in the Arduino source code).

ATmega8/168/328p pinout diagram

The only pins that I needed to consider were:

Step 3: Connecting via AVRdude

AVRdude is an open-source utility for programming Atmel AVR Microcontrollers.

Running the following from a terminal

avrdude -p atmega8 -c avrisp -P /dev/ttyUSB0 -b 19200

produced

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.13s

avrdude: Device signature = 0x1e9307

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

Which indicated that AVRdude had communicated  with the mega-isp successfully.